In the production of industrially relevant target products, such as, for example, (meth)acrylic acid, aqueous compositions which also contain numerous impurities in addition to the desired target products and water are frequently obtained in the course of the production. Before (meth)acrylic acid can be used as starting material in the preparation of polymers by free radical polymerization, it was customary to date to isolate the (meth)acrylic acid from such aqueous compositions, a purity of more than 99% by weight frequently being strived for in the case of acrylic acid. In the preparation of poly(meth)acrylates from a high-purity (meth)acrylic acid obtained in this manner, the latter is then dissolved in water as a solvent and is polymerized in the presence of free radical initiators.
Crystallization may be mentioned as one possibility for the preparation of high-purity, organic substances, such as, for example, high-purity acrylic acid. In particular, two processes are used industrially, suspension crystallization and layer crystallization (Wintermantel et al., Chem. Ing. Tech. 1991, 63, 881-891; Steiner et al., Chem. Ing. Techn. 1985, 57, 91-102).
However, one crystallization step alone is often not enough for removing byproducts sufficiently well out of or from the crystals since microinclusions of mother liquors or the incorporation of impurities at crystal defects, etc. cannot be ruled out under finite crystal growth conditions. In particular, the adhesion of mother liquor to the crystal can adversely affect the purity of the products.
For this reason, the crystals produced are frequently, particularly in the case of a crystal suspension, washed with wash liquids after the separation from the mother liquor and/or, in the case of the layer or suspension crystallization, the crystals are subjected to a sweating or wash process in which impurities of any type can, if required, be depleted.
Such a process can be carried out continuously in a so-called wash column. An overview in this respect is given in the dissertation by Poschmann (Zur Suspensionskristallisation organischer Schmelzen und Nachbehandlung der Kristalle durch Schwitzen und Waschen [On the suspension crystallization of organic melts and aftertreatment of the crystals by sweating and washing], thesis, Uni. Bremen, Shaker Verlag, Aachen 1996).
Such a process in which (meth)acrylic acid crystals are crystallized out of an aqueous melt comprising (meth)acrylic acid, water and impurities differing from (meth)acrylic acid and water and the (meth)acrylic acid crystals thus obtained are at least partly washed with a crystal melt is described, for example, in WO-A-03/078378. According to the teaching of this publication, (meth)acrylic acid is crystallized from such an aqueous solution at a temperature in the range of −5 to 30° C., the crystals thus obtained are then melted at a temperature in the range from 10 to 15° C. and the (meth)acrylic acid crystals are then washed with the crystal melt.
However, the disadvantage of the process described in WO-A-03/078378 is, inter alia, that the crystal bed obtained there tends to a high degree of compaction so that it is necessary to use hydraulic pressure which is problematically high for industrial use, in particular with the use of continuously operated wash columns. Furthermore, the process described in WO-A-03/078378 has the disadvantage that a comparatively large amount of energy has to be applied for melting the (meth)acrylic acid.